On 20/05/2017 17:44, ChrisK wrote:
There is a noticable issue with HW water flow rate upstairs &
interaction between showers & filling the bath is a bit slow. This is
probably down to re-using the original 15mm pipe work. It would have
been better to have replaced this with 22mm.

If your mains pressure and flow rate are adequate then 15mm should be
fine. Sounds like you have another problem somewhere.